1. Field of the Invention
The invention relates generally to imaging systems and in particular to imaging scenes onto two-dimensional focal plane arrays in two or more wavebands.
2. Description of the Prior Art
A new generation of focal plane array technologies permit imaging in two or more wavebands using a single focal plane array. In the infrared portion of the spectrum, the wavebands are typically in the medium wavelength infrared (MWIR) band at 3-5 .mu.m and in the LWIR band of 8-12 .mu.m. (See for example Reine et al., Independently Accessed Back-to-Back HgCdTe Photodiodes: A New Dual-Band IR Detector, Journal of Electronic Materials, Vol. 24, No. 5, 1995.) These multi-band focal plane array systems enjoy a time-multiplex advantage over systems where selectable spectral filters are required to perform this function.
Infrared imaging typically requires a focal plane array (FPA) operating at cryogenic temperatures. To do this, the FPA is mounted in a Dewar or similarly cooled enclosure. The cryogenic environment also serves to eliminate extraneous thermal radiation originating from the optical system structural members located outside the cryogenic enclosure. Cold pupil stops are also commonly employed in long-wavelength infrared (LWIR) sensors to further reduce extraneous emissions of LWIR background radiation, i.e., all radiation other than that collected by the sensor aperture and within the focal plane array's field of view. The location of the pupil stop, as is known in the art, depends on the precise optical prescription and is generally located between a cold field stop and the FPA.
The point-spread function (PSF) of diffraction-limited optical systems widens with increasing wavelength. Consequently, for dual-band systems, the sensor point spread function tends to be over-sampled at the longer passband where the effects of diffraction are greater, and under-sampled in the shorter bandpass, by the pixels in the focal plane array. The same geometric centers obtain for the two wavebands for all current dual-band FPAs. For dual-band IR sensors systems, for example, the PSF results in two different resolutions for an ordinary cold pupil stop and fewer pixel samples for the PSF for the MWIR band than for the LWIR band.